The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor or augmentor. A common solution is to protect the surfaces of such components with an environmental coating system, such as an aluminide coating or a thermal barrier coating system (TBC). The latter includes an environmentally-resistant bond coat and a layer of thermal-insulating ceramic applied over the bond coat. Bond coats are typically formed from an oxidation-resistant alloy such as MCrAlY where M is iron, cobalt and/or nickel, or from a diffusion aluminide or platinum aluminide that forms an oxidation-resistant intermetallic. Metal oxides, such as zirconia (ZrO.sub.2) that is partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or another oxide, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure.
Bond coats formed with the above-noted compositions protect the underlying superalloy substrate by forming an oxidation barrier for the underlying superalloy substrate. The aluminum content of these bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the ceramic layer to the bond coat. Though bond coat materials are particularly alloyed to be oxidation-resistant, the oxidation that occurs over time at elevated temperatures gradually depletes aluminum from the bond coat. Eventually, the level of aluminum within the bond coat is sufficiently depleted to prevent further slow growth of the protective oxide, and to allow for the more rapid growth of nonprotective oxides. At such time, spallation may occur at the interface between the bond coat and the aluminum oxide layer or the interface between the oxide layer and the ceramic layer. Even without the formation of nonprotective oxides, spallation may occur due to stress generation.
Spallation of the ceramic layer is often triggered by the differences in coefficients of thermal expansion of the superalloy substrate, metallic bond coat and ceramic layer, including the thermally-grown aluminum oxide layer. As represented in FIG. 1, a thermal barrier coating system is shown as comprising a ceramic layer 12 adhered to a substrate 10 by a bond coat 14. The coefficients of thermal expansion (CTE or .alpha.) of the substrate 10 and metallic bond coat 14 are roughly equal, as are their coefficients of thermal conductivity (k). However, the CTE and thermal conductivity of the ceramic layer 12 are considerably less than that of the substrate 10 and bond coat 14. For example, the CTEs of ceramic materials used to form the ceramic layer 12 are generally on the order of about 50%-60% of that of the materials for the substrate 10 and bond coat 14. The CTE of the protective oxide layer is even lower than that of the ceramic layer 12. Consequently, and as represented in FIG. 1, while little relative expansion occurs at the interface 16a between the substrate 10 and bond coat 14 at elevated temperatures, a considerable difference in expansion occurs at the interface 16b between the bond coat 14 and ceramic layer 12. This difference in expansion generates considerable shear forces that promote spallation of the ceramic layer 12.
As is evident from FIG. 1, the maximum service temperatures of the substrate 10 (T.sub.2), bond coat 14 (T.sub.3) and the ceramic layer 12 (T.sub.4) also differ from each other due to their differences in thermal conductivity. Notably, the temperature T.sub.4 at the outer surface of the ceramic layer 12 is considerably higher than the temperature T.sub.3 at the interface 16b between the ceramic layer 12 and bond coat 14. The lower service temperature of the bond coat 14 reduces its rate of oxidation, and therefore promotes the overall service life of the coating system.
To reduce the difference in thermal expansion between the ceramic layer and bond coat of a thermal barrier coating system, graded bond coats have been proposed in the prior art. An example of such a coating system is represented in FIG. 2, which shows a bond coat 14 composed of inner and outer layers 14a and 14b. The conventional practice has been to formulate the inner and outer layers 14a and 14b to have CTEs between that of the substrate 10 and ceramic layer 12, with the CTE of the inner layer 14a being closer to that of the substrate 10 and the CTE of the outer layer 14b being closer to that of the ceramic layer 12. For example, the inner layer 14a may have a composition of about two parts bond coat alloy and one part metal oxide, while the outer layer 14b would have a composition of about one part bond coat alloy and two parts metal oxide. The resulting advantageous "graded" effect on thermal expansion is schematically and graphically represented in FIG. 2.
Also shown in FIG. 2 is the effect that the graded bond coat composition has on service temperature. Notably, the bond coat layers 14a and 14b have lower coefficients of thermal conductivity as compared to the bond coat 14 of FIG. 1 due to their inclusion of metal oxides, whose coefficients of thermal conductivity are considerably lower than that of metallic bond coat alloys. Because the bond coat layers 14a and 14b cannot conduct heat as readily to the substrate 10, the service temperature of the bond coat 14 is higher, as shown by the indicated temperatures T.sub.3a and T.sub.3b for the interfaces 16b and 16c between the inner and outer bond coat layers 14a and 14b, and between the outer layer 14b and the ceramic layer 12, respectively. Accordingly, while the graded bond coat composition of FIG. 2 reduces dissimilarities in thermal expansion, the higher service temperature of the bond coat 14 (often on the order of about a 10.degree. C. difference) leads to accelerated oxidation, thus shortening the service life of the coating system.
In view of the above, it can be seen that, while graded bond coat compositions of the past promote the service life of a thermal barrier coating system in one respect, the resulting increase in oxidation rate of the bond coat has a converse effect. Furthermore, the combination of metal and ceramic in graded bond coats produces a bond coat of limited ductility and toughness at the service temperatures encountered in a gas turbine engine. Accordingly, what is needed is a bond coat that yields a gradation of thermal expansion between the substrate and ceramic layer of a thermal barrier coating, without raising the service temperature of the bond coat. Such a bond coat would also preferably exhibit ductile behavior over a large portion of its service temperature range to allow for stress relaxation.